In Situ Generated ABA Block Copolymers from CO2, Cyclohexene

Letter pubs.acs.org/macroletters

In Situ Generated ABA Block Copolymers from CO2, Cyclohexene Oxide, and Poly(dimethylsiloxane)s Marina Reiter, Alexander Kronast, Stefan Kissling, and Bernhard Rieger* WACKER-Lehrstuhl für Makromolekulare Chemie, Technische Universität München, Lichtenbergstraße 4, 85748 Garching, Germany S Supporting Information *

ABSTRACT: Chain-transfer polymerization reactions with siloxanes, CO2, and cyclohexene oxide have been conducted, utilizing two β-diiminate (BDI) zinc-based catalysts, BDICF3(1)-ZnEt and BDI CF3 (2)-ZnEt ((BDI CF3 (1))H = [CH(CCF 3 NC 6 H 4 -2,6C 2 H 5 ) 2 ] and (BDI CF3 (2))H = [CH(CCF 3 NC 6 H 4 -2,6-CH(CH3)2)2]). The correlation between equivalents of siloxane and the corresponding molecular masses and glass transition temperatures is exhibited. Furthermore, the in situ preparation of ABA block copolymers from carbon dioxide, cyclohexene oxide, and α,ω-bis(hydroxymethyl)poly(dimethylsiloxane)s is presented. This reaction was found to strongly relate to a robust Lewis acid catalyst like the outlined complexes. The polymer properties can be tuned by varying the amount of chain-transfer agent or changing the catalyst. The resulting polymer structures and incorporation of siloxanes were revealed by 29Si NMR spectroscopy, 1 H NMR spectroscopy, ESI-MS, GPC, and DSC.

A

Chart 1. Catalysts 1−4 for Copolymerization of CO2 and Cyclohexene Oxide

liphatic polycarbonates, derived from carbon dioxide and epoxides, have drawn considerable attention in recent decades owing to their durability, biodegradability, high transparency, heat resistance, and gas permeability.1−5 Thus, they are proposed as alternatives to petrochemicals in automotive, medical, and electronics industries. Poly(cyclohexene carbonate) (PCHC), especially, is brittle and has a high glass transition temperature (Tg).6,7 To modify the Tg, decomposition temperature and brittleness, terpolymerization of different epoxides and carbon dioxide,8−13 and ringopening polymerization of lactides or esters combined with copolymerization of epoxides/CO2 were studied.14−19 However, chain-transfer polymerization is a promising way to connect different types of polymers to create polycarbonate polyols with defined molecular weights and different material characteristics.20−24 Low-molecular-weight polycarbonates with terminal hydroxyl groups can be used, for example, in polyurethane production. These polymers are used in the manufacturing of foams, adhesives, coatings, and synthetic fibers.24,25 In a key study, in situ formation of ABA or AB block copolymers from propylene oxide, CO2, and chain-transfer agents such as adipic acid, poly(ethylene glycol), poly(propylene glycol), or polycaprolactone, catalyzed by an ionic cobalt salen complex, was shown. 21 Focusing on the copolymerization of cyclohexene oxide and CO 2 , no dihydroxy-terminated molecules or polymers have been reported as chain-transfer agents to the best of our knowledge (only small molecules, e.g., water or ethanol).22,26 The most active catalyst for the production of PCHC relies on a βdiiminate (BDI) ligand and zinc as metal center (Chart 1).27−29 As the main disadvantage of PCHC is brittleness, the intention was to incorporate poly(dimethylsiloxane)s (PDMSs) into PCHC. PDMSs have a very low glass transition temperature of © XXXX American Chemical Society

about −120 °C, are stable over a broad temperature range and under UV light, and are nontoxic and hydrophobic.30−32 Early attempts to combine polycarbonate and PDMS were achieved solely via postmodification of poly(bisphenol A carbonate) (PC) with PDMS or the synthesis of PC−PDMS blends.33,34 Herein, we report the chain-transfer polymerization of carbon dioxide, cyclohexene oxide, and polysiloxanes with BDI-Zn catalysts. The resulting polycarbonate polyols were characterized by NMR spectroscopy, electrospray ionization mass spectroscopy (ESI-MS), gel-permeation chromatography (GPC), and differential scanning chromatography (DSC). In general, PDMSs are known to be sensitive to backbiting or combination reactions (condensation) with other PDMS chains, forming water as a side product. As BDI-Zn catalysts decompose in the presence of water, an alternative to common Received: February 17, 2016 Accepted: March 4, 2016

419

DOI: 10.1021/acsmacrolett.6b00133 ACS Macro Lett. 2016, 5, 419−423

Letter

ACS Macro Letters PDMS must be found. Recently, the synthesis of α,ωbis(hydroxymethyl)-poly(dimethylsiloxane)s, called carbinoltelechelic PDMSs, was presented by our group via reaction of bis(hydroxyl)-terminated PDMSs with equimolar amounts of 2,2,5,5-tetramethyl-1,4-dioxa-2,5-disilacyclohexane (Scheme 1).35 In contrast to bis(hydroxyl)-terminated PDMSs,

Scheme 2. Synthesis of Siloxane-Terminated Copolymers and PCHC/PDMS Triblock Copolymers

Scheme 1. Synthesis of α,ωBis(hydroxymethyl)poly(dimethyl-siloxane)s with 1,1,3,3Tetramethylguanidine (TMG) as Catalyst

carbinol-telechelic PDMSs are stable over a long-term and do not tend to form cyclic siloxanes or water via backbiting or recombination reactions. The formation of PCHC is predominantly catalyzed by zincbased catalysts, where the ligands rely on phenoxides, βdiiminates, or macrocyclic Robson-type structures.27,28,36−38 Particularly, BDI-Zn catalysts exhibit higher activities for the coupling reaction of CO2 and epoxides if the Lewis acidity of the zinc center increases by introducing electron-withdrawing groups into the ligand backbone.27,39 Recently, we described the synthesis of BDICF3(1)-ZnEt and BDICF3(2)-ZnEt (Chart 1) and their activity toward the ring-opening polymerization of (rac)-butyrolactone and (rac)-lactide.40 For the first time, these catalysts were tested for copolymerization of CHO and CO2 by performing experiments in an in situ IR autoclave (Figure S1 and Table S1). Catalyst 1 yielded a TOF of 4800 h−1, a selectivity for carbonate linkages of 91%, and molecular weights up to 102 kg/mol with a narrow PDI of 1.3 (Table S1, entry 2). Hence, catalyst 1 is highly effective for the formation of PCHC. In contrast, catalyst 2 was found to be less active with a TOF of 570 h−1, with the formation of ether linkages being favored. A molecular weight of 49 kg/mol was obtained with a PDI of 2.2 (Table S1, entry 1). However, the analogue to complex 2 without electron-withdrawing CF3 groups was completely inactive for copolymerization of CO2 and CHO.37 As complex 2 promotes formation of ether linkages, homopolymerization of CHO catalyzed by 2 (100 °C, 1 h, CHO/2, 1000:1) was also performed. Poly(cyclohexene oxide) with a Mn of 51 kg/mol and a TON of 50 was obtained. To verify that hydroxymethyl siloxanes act as chain-transfer agents in the copolymerization of CO2 and cyclohexene oxide, molecule A was synthesized by reaction of trimethylsilanol and 0.5 equiv of 2,2,5,5-tetramethyl-1,4-dioxa-2,5-disilacyclohexane. The results of the copolymerizations with molecule A (Scheme 2) are summarized in Table 1. Complexes 1 and 2 are able to catalyze this chain-transfer polymerization under appropriate conditions (pressure, solvent, time, and temperature were adjusted for the respective catalyst). As reference systems, the dinuclear zinc-based catalyst 3 and the monometallic zinc-based catalyst 4 were synthesized according to literature28,41 and tested in chain-transfer polymerizations with siloxane A. Particularly, catalysts 1 and 3 exhibited good yields and high selectivity for carbonate linkages. Therefore, the amount of A was varied from 10 to 30 or 10 to 80 equiv depending on the catalyst. All obtained polymers have a narrow monomodal GPC distribution curve. The molecular weights decrease as the [A]/ [catalyst] ratio increase, as expected for an immortal polymerization. Mn was measured by GPC and was consistent with

molecular weights determined by 1H NMR spectroscopy. ESIMS studies, 29Si NMR spectroscopy, and 1H NMR spectroscopy of a low-molecular-weight polymer (Table 1, entry 5) revealed the polymers’ molecular structure, which is presented in Scheme 2 (Figures S2−S4). To determine whether increasing the chain length between the siloxane unit and hydroxyl function influences the chain-transfer polymerization, molecule B with an n-propyl linkage between the siloxane unit and the OH terminus was synthesized by a hydrosilylation reaction of allyl alcohol and pentamethyldisiloxane with Karstedt’s catalyst.42 Copolymerization of CHO, CO2, and 20 equiv of B (compared to the catalyst) resulted in a polymer terminated by B with a molecular weight of about 6900 g/mol (Table 1, entry 6). This value is similar to the results of copolymerization of CHO and CO2 and 20 equiv of A (Table 1, entry 2), indicating that A and B are effective chain-transfer agents. As the preliminary tests utilizing siloxanes A and B were successful, copolymerization experiments with catalysts 1 and 2 (Table 2) and the dihydroxy-terminated carbinol-telechelic PDMS were performed. The polysiloxane has a molecular weight of about 1300 g/mol according to 1 H NMR spectroscopy. With increasing amounts of carbinol-telechelic PDMS, a trend to lower molecular weights was observed, which provides the opportunity for a controlled polymer synthesis with a defined molecular weight. Both methods for the determination of molecular weight, NMR spectroscopy and GPC, are subject to error; therefore, in Table 2, both values are listed in each case. The selectivity for carbonate or ether linkages differed, catalyst 1 reaching up to 98% carbonate content (Table 2, entry 4), whereas catalyst 2 forms up to 70% ether linkages (Table 2, entry 9). Further polymer analysis by DSC showed that low-molecular-weight polymers in particular (Table 2, entries 4−6, 9) had extremely low Tgs. An ABA block NMR copolymer with a molecular weight of 2800 g/mol showed a Tg of 37 °C (Table 2, entry 4). Glass transition temperatures are known to strongly depend on the molecular weight of a polymer. However, a Tg of 85 °C for PCHC terminated by A with almost the same molecular weight of 2500 g/mol (Table 1, entry 4) could be determined. This is a difference of almost 50 °C, which can clearly be attributed to the presence of carbinol-telechelic PDMS. Where the polymer had some ether content, the difference was almost 100 °C, although molecular masses were in the same range (Table 2, entry 9). Additionally, no phase separation was observed, as only one Tg was found for all the polymers via DSC. DSCs of PCHC−PDMS blends showed always two different Tgs at 420


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ACS Macro Letters

Table 1. Chain-Transfer Polymerizations with CO2/CHO/Siloxane A (Entries 1−5, 7−12) or CO2/CHO/Siloxane B (Entry 6) Catalyzed by Complexes 1−4 entry

catalyst

chain-transfer agent

[CTA]/ [cat]

selectivitye (%)

TONf

Mng (g·mol−1) (GPC)

Mnh (g·mol−1) (1H NMR)

PDIg

Tgi (°C)

1a 2a 3a 4a 5a,j 6a 7b 8c 9c 10c 11c,j 12d

1 1 1 1 1 1 2 3 3 3 3 4

A A A A A B A A A A A A

10 20 40 60 80 20 10 10 20 25 30 10

96:04:00 96:04:00 95:05:00 94:06:00 92:08:00 95:04:01 53:44:01 93:07:00 99:01:00 99:01:00 82:18:00 90:00:10

650 700 750 750 800 700 330 900 900 900 50 150

12100 6900 3500 2400 1300 6900 3300 25500 12200 8800 n.d. 4800

13000 8900 4000 2500 1800 7000 3300 23000 12700 8800 600 5400

1.1 1.2 1.2 1.2 1.2 1.3 1.2 1.5 1.3 1.2 n.d. 1.2

102 97 93 85 66 n.d.k 75 107 111 106 n.d. 105

Reaction conditions: 40 bar, 50 °C, 1000:1, neat CHO, 1 h. bReaction conditions: 40 bar, 100 °C, 1000:1, 2.5 mL toluene, 2 h. cReaction conditions: 30 bar, 100 °C, 1000:1, 2.5 mL toluene, 1 h. dReaction conditions: 30 bar, 100 °C, 500:1, 2.5 mL toluene, 2 h. e[PCHC]/[PCHO]/ [cCHC], assigned by the relative integrals of the signals at δ = 4.65 ppm (polycarbonate), δ = 3.5−3.6 ppm (polyether), and δ = 4−4.08 ppm (cyclic carbonate). fThe turnover number (TON) is the ratio of the number of moles of epoxide consumed to the number of moles of catalyst. gDetermined by GPC, calibrated with polystyrene standards in tetrahydrofuran. hDetermined by 1H NMR spectroscopy. iGlass transition temperature obtained from DSC. jNot precipitated due to good solubility in MeOH. kn.d. = not determined. a

Table 2. Synthesis of ABA Block Copolymers from CO2/CHO/α,ω-bis(hydroxymethyl)poly(dimethylsiloxane)s Catalyzed by Complex 1 (Entries 1−6) and Complex 2 (Entries 7−9)a entry catalyst 1 2 3 4 5 6h 7 8 9h

1 1 1 1 1 1 2 2 2

chain-transfer agent

[CTA]/[cat]

selectivityb (%)

TONc

Mnd (g·mol−1) (GPC)

Mne (g·mol−1) (1H NMR)

Mnf (g·mol−1) (theoretical)

PDId

Tgg (°C)

C C C C C C C C C

5 10 20 30 40 50 5 10 20

96 95 96 98 88 93 46 43 30

700 750 800 500 400 150 650 650 300

18000 10300 5800 2700 1200 1200 18300 8500 2600

22000 12000 6500 3800 2700 1500 n.d.i n.d. n.d.

17900 11000 6700 3600 2700 1700 n.d. n.d. n.d.

1.2 1.2 1.2 1.3 1.3 1.5 1.3 1.2 1.2

107 91 80 37 20 −87 86 67 −12

Reaction conditions: 40 bar, 50 °C, 1000:1, neat CHO (2.5 mL), 1 h. bSelectivity for polycarbonate, which is assigned by the relative integrals of the signals at δ = 4.65 ppm (polycarbonate), δ = 4−4.08 ppm (cyclic carbonate), and δ = 3.5−3.6 ppm (polyether); no cyclic carbonate was formed in these block copolymerization reactions. cThe TON is calculated by the number of moles of consumed epoxide divided by the moles of catalyst. d Determined by GPC, calibrated with polystyrene standards in tetrahydrofuran. eDetermined by 1H NMR spectroscopy (3.90−3.65 ppm corresponds to an integral of 4H (−OCH2Si(Me)2O−)). fCalculated: Mn = [(1000: ([CTA C] + 1))·142 g/mol · (TON:1000)] + 1300 g/mol. g Glass transition temperature obtained from DSC. hNot precipitated due to good solubility in MeOH. in.d. = not determined. a

Figure 1. ESI-MS of ABA polycarbonate-polysiloxane-polycarbonate polymer (Table 2, entry 6). 421


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ACS Macro Letters −120 °C for PDMS and 50−100 °C for PCHC, depending on the molecular weight of the polycarbonate (Table S2). However, it remains to be proven that these material properties were not caused by a polymer blend of PCHC and PDMS. Hence, ESI-MS investigations were performed (Figure 1), which showed that the oligomer series comprises [17(OH) + 142n (repeating unit PCHC) + 14(CH2) + 74n (repeating unit polysiloxane [SiMe2O]n) + 72 (SiMe2CH2) + 142n (repeating unit PCHC) + 17(OH) + 23(Na +)]. The carbinol-telechelic PDMS did not have a single repeating unit but showed a molecular weight distribution (n = 6−11) in the ESI-MS. Hence, there are signals for different PCHC units (repeating unit of 142 g/mol) visible for an ABA block copolymer and additional signals for different chain lengths of the carbinol-telechelic PDMS (Figure 1, blank circles). Figure 1 is clear evidence that the polymer is 2-fold hydroxyl-terminated and is an ABA block copolymer. In the 1H NMR spectrum (Figure S5), the terminal CH2 group of the carbinol-telechelic PDMS in the ABA block copolymer (Table 2, entry 6) could be clearly assigned to a multiplet from 3.89 to 3.79 ppm (this was confirmed by COSY and HSQC NMR, Figures S6 and S7). The shift of this CH2 group from 3.24 ppm (original PDMS) to 3.89−3.79 ppm (PDMS in the polymer) indicates that the linkage between the polysiloxane and the polycarbonate must be a carbonate and not an ether unit (Figure S5), which was verified by ESI-MS (Figure 1). Additionally, in the 29Si NMR spectra, the signal for the terminal silicon of PDMS at 3.27 ppm disappeared and a new signal appeared at 1.57 ppm, which could be attributed to the incorporated PDMS (Figure S8). Incorporation of the carbinol-telechelic PDMS was thus confirmed via 29Si, 1H, COSY, and HSQC NMR spectroscopy, DSC, GPC, and ESI-MS. Unexpectedly, reference systems 3 and 4 showed no activity for this chain-transfer polymerization, as no polymer at all was formed. Only catalysts 1 and 2 could produce the desired block copolymers in situ from CO2, cyclohexene oxide, and the carbinol-telechelic PDMS. This lack of activity by catalysts 3 and 4 was further investigated via NMR spectroscopy (Figures S9−S11). About 10 mg of catalyst 4 was dissolved in 0.5 mL of CDCl3, and after adding an excess of carbinol-telechelic PDMS, 1 H NMR was performed (Figure S9, 5 min, 30 min, 2.5 h, 19 h). Complex 4 immediately reacted with the polysiloxane and two new species were formed. After 19 h, the complex decomposed to the ligand and an unknown species. This can be attributed to a stable and unreactive molecule with PDMS bound to a zinc center instead of an N(TMS)2 group, as no signals remained to be attributed to a zinc-coordinated N(TMS)2 group. This experiment was also undertaken with a previously reported BDI(CH3)(2)-ZnEt catalyst, which is inactive for copolymerization of CO2/CHO but has an ethyl group bound to the zinc center, as in complexes 1 and 2 (Figure S13).37 In this case, again, an unknown species was formed, and after 20 h, the solution consisted of free ligand, the original catalyst, and an unidentified species (about 60%). When the same experiment was repeated with complex 3, it was found that after adding an excess of carbinol-telechelic PDMS, the whole solution instantly formed a solid block, insoluble in all tested solvents. Probably, the dinuclear catalyst “polymerized” with the carbinol-telechelic PDMS, as both components have two reactive sites (Figure S12). Only complex 2 did not interact with the polysiloxane, even after 1 day (Figure S11). Complex 1 started to decompose to the

corresponding ligand (due to protic impurities), but did not interact further with the polysiloxane (Figure S10). In summary, we successfully tested catalysts 1 and 2 in the copolymerization of CO2 and CHO. It was shown that a onepot synthesis of ABA diblock copolymers from CO2, CHO, and carbinol-telechelic PDMS is possible. This reaction is only catalyzed by catalysts 1 and 2, as confirmed via 1H NMR measurements. The obtained low-molecular-weight polycarbonate-polyols exhibited very low glass transition temperatures, which is clearly a result of the amount of integrated PDMS. Furthermore, the incorporation of the polysiloxane was confirmed using several analytical methods, such as 29Si, 1H, COSY, HSQC NMR spectroscopy, and ESI-MS. Finally, we could determine the molecular structure of the obtained polymers by the mentioned analytical methods. The fact that these ABA block copolymers are 2-fold hydroxyl-terminated offers possibilities for further research in industrial applications such as the production of polyurethanes.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.6b00133. Experimental procedures and further analytical data and characterization data for polymers (PDF).

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Richard O. Reithmeier and Peter T. Altenbuchner for support with the preparation of this paper. REFERENCES

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In Situ Generated ABA Block Copolymers from CO2, Cyclohexene

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